The conversion of radio frequency signals to higher or lower frequencies has been done in the past in one of two ways: mixing an input radio frequency signal with a local oscillator signal, thereby obtaining sum and difference signals (heterodyning), or using electrically non-linear devices, thereby generating harmonic or sub-harmonic signals.
A problem inherent in the heterodyne conversion technique using a local oscillator signal is that the absolute bandwidth remains unchanged, and the fractional bandwidth increases. For instance, an input signal frequency band of (f.sub.2 -f.sub.1) with center frequency f.sub.0 will retain the same bandwidth even though the center frequency has been reduced by n to f.sub.0 /n or increased by n to nf.sub.0. In the down conversion process, the ultimate instantaneous bandwidth is clearly limited.
A problem which is inherent in the non-linear device conversion technique is that the converted signal can only be an integral multiple or sub-multiple of the input frequency.
Also, both heterodyning and harmonic/sub-harmonic conversion are non-linear processes which create undesirable harmonic signals and intermodulation products of multiple input signals.